How to pour a concrete slab for a shed (with quantities)

Before you start

Three checks: the local rules (permits, setback from the boundary, frost depth in cold climates), where the services run (call the locating service before any digging), and where the water goes. A shed slab should sit an inch or two above the surrounding ground and the ground should fall away from it, not towards it.

Tools for a small slab: a spade and a wheelbarrow, a plate compactor (hire it), timber for the forms with stakes and screws, a long straight board for screeding, a wooden or magnesium float, an edging tool, a level and a string line. For bags you will want a mixer; for a truck, a second pair of hands and a rake.

Choose the thickness

Four inches is the standard shed slab and it is plenty for a garden shed, bikes and a workbench. Six inches with mesh or rebar is for a workshop with heavy machines, a ride-on mower or a motorbike, and for anything a building inspector is going to look at. Thicker means half as much again in concrete, so decide this first.

How much you need: concrete and gravel for common shed sizes

Every figure comes from the concrete calculator and the gravel calculator with the same waste allowances they use: 10% on the concrete, 5% on the gravel, 80 lb bags at 0.60 cubic feet each and gravel at 1.5 tonnes per cubic metre. Change any dimension in the calculator and you get your own numbers.

The 10 x 10 slab is the classic bag-or-truck case; the 10x10 slab guide lists it in every bag size, and bagged versus ready-mix explains where the break-even sits.

Step 1: excavate and lay the gravel base

Mark the slab with string and stakes a foot bigger than the shed each way, then dig out the topsoil to 8 inches below the finished slab level: 4 inches of gravel plus 4 inches of concrete. Level the bottom of the hole, compact it, then spread the crushed stone in two layers and compact each one with the plate compactor until it stops moving under the machine. A base you can walk on without leaving footprints is ready.

Step 2: build and level the forms

Two-by-four timber on edge gives a 3.5 inch slab; use 2x6 and set it to the height you want for a true 4 inches. Stake the boards every 2 to 3 ft on the outside, screw them to the stakes, and check the diagonals: a rectangle is square when both diagonals measure the same. Set the top of the forms to a level line with a slight fall of about 1/8 inch per foot if you want water to run off, and oil the inside face so the boards come away cleanly.

Step 3: reinforcement and the last check

Welded wire mesh on small chairs in the middle of the slab thickness stops shrinkage cracks from opening; for a 6 inch workshop slab use rebar on a grid instead. Lay a plastic vapour barrier over the gravel if the shed floor will be finished or heated. Then measure the inside of the forms one last time and run the numbers again: the concrete is ordered on those dimensions, and a slab that is half an inch thicker than planned everywhere is a tenth more concrete.

Step 4: mix or take delivery, and pour

With bags, have every bag on site, mix to the water quantity printed on the bag and keep the mixer going; the slab has to be poured in one session so it cures as one piece. With a truck, agree the time, have the barrow route ready, and pour from the far end towards the truck. Rake the concrete level just above the forms, then screed: rest a straight board on the forms and saw it back and forth across the slab, pulling the surplus in front of it.

Step 5: finish

Once the surface water has gone, float the slab to close the surface and push the stones down, run the edging tool around the forms, and either broom it for grip or trowel it smooth if the shed floor will be covered. Cut a control joint across the middle of any slab over about 10 ft long, either with a groover while it is soft or with a saw the next day, so the shrinkage crack happens in the joint and not across the floor.

Step 6: cure it, do not dry it

Concrete gains strength by staying damp. Cover the slab with plastic sheeting or a curing blanket, or mist it with water twice a day, for the first week. In hot sun an uncovered slab loses its water in hours and stays weak and dusty. Keep off it for a day, walk on it after two, strip the forms after two or three, and build the shed after about a week.

Building slabs for other people?

The calculators give you the material list. The Pro quote builder turns that list into a quote with your labour, extras, margin and tax, printed with your logo, so a shed base is priced in the time it takes to measure it.